Wireless communication techniques based on multiple subcarriers, such as an orthogonal frequency-division multiplexing (OFDM) technique, are gaining worldwide popularity due to their broad applications. For example, an OFDM based communication system may be used in a plurality of networks including Worldwide Interoperability for Microwave Access (WiMax) networks, Wireless Fidelity (Wi-Fi) networks, Wireless Broadband (WiBro) networks, etc.
The OFDM technique uses a plurality of closely-spaced orthogonal subcarriers to carry data. For example, the data may be allocated on a plurality of parallel data channels, one for each of the subcarriers. Each of the subcarriers may be modulated with a conventional modulation scheme, e.g., quadrature amplitude modulation, at a relatively low symbol rate. In addition, based on the OFDM technique, an inverse fast Fourier transform (IFFT) may be performed on OFDM symbols representing the data on a transmitter side, and a fast Fourier transform (FFT) may be performed to recover the OFDM symbols on a receiver side. Signals including the OFDM symbols are transmitted from the transmitter side to the receiver side through a communication channel.
In reality, the communication channel may have an effect on the signals when the signals are transmitted. The receiver side may need knowledge of the communication channel to remove such effect, in order to accurately recover the data. To facilitate estimation of the communication channel, signals known to both the transmitter side and the receiver side, i.e., pilot symbols, may be inserted in OFDM symbols on the transmitter side. The receiver side may perform channel estimation based on resource blocks in received signals, and each of the resource blocks includes a plurality of OFDM symbols and, hence, pilot symbols.
FIG. 1 illustrates a structure of a conventional resource block 100 in a time-frequency domain. For example, the resource block 100 includes data to be transmitted by an OFDM based communication system having first, second, third, and fourth antennas transmitting first, second, third, and fourth data streams, respectively. The data to be transmitted are carried by a plurality of subcarriers, one corresponding to each row in FIG. 1.
The resource block 100 includes a plurality of data symbols each represented by a small block with a letter “D” and a plurality of pilot symbols each represented by a small block with an indexed letter P. For example, the small blocks with indexed letters “P1,” “P2,” “P3,” and “P4” represent the pilot symbols for the first, second, third, and fourth data streams, respectively. The data and/or pilot symbols at a same time, i.e., in a same column in FIG. 1, correspond to an OFDM symbol. In FIG. 1, the resource block 100 includes six OFDM symbols S1, . . . , S6.
The receiver side may perform channel estimation based on the pilot symbols in the resource block 100. For example, based on the pilot symbols labeled as “P1”, which correspond to ones of the plurality of subcarriers, the receiver side may use interpolation techniques to estimate a channel response at frequencies of all of the plurality of subcarriers for the first data stream.
However, the pilot symbols for the first data stream, i.e., the pilot symbols labeled as “P1,” are not uniformly distributed in frequency, as indicated in FIG. 1 by the number “2” or “3” for a frequency spacing between the pilot symbols for the first data stream. As a result, accuracy of channel estimation may be degraded for the first data stream. Similarly, accuracy of channel estimation may also be degraded for the second data stream.